1. Field of the Invention
The present invention relates generally to a cleaning liquid and method to clean metal masks for improving print consecutiveness in surface mounting technology. More specifically, it relates to an improved cleaning liquid and method to wash metal masks where solder cream is applied to form patterns therethrough on the pads of printed circuit boards(PCBs), reducing the rate of poor PCBs and extending cleaning period for metal masks.
2. Description of the Related Art
In the solder printing process of the surface mounting technology, solder cream is applied to a metal mask having the same pattern holes as a predetermined circuit pattern to be printed on the pads of PCBs and the solder cream passes through the holes to form the predetermined circuit pattern on the pads of the PCBs. However, since the width of the holes are so minute, the holes are prone to be blocked up and this results in poor PCBs.
The two Asano et al. patents, U.S. Pat. Nos. 5,302,313 and 5,271,775, respectively entitled Halogenated Hydrocarbon Solvents, and Methods For Treating Substrates By Applying A Halogenated Hydrocarbon Thereto, each disclose a solvent composition consisting of a fluorine compound combined with an organic solvent which may be alcohol. The Buchwald et al. patent, U.S. Pat. No. 5,271,861, entitled Fluorochlorohydrocarbon-Free Cleaning Compositions, discloses a cleaning composition composed of alcohol or partially fluorinated alcohol and a surfactant. The Rozen et al. patent, U.S. Pat. No. 5,250,213, entitled 1,1,1,2,2,3,3,4,4,5,6-Undecafluorohexane And Use Thereof In Compositions And Processes For Cleaning, discloses the combination of a fluorine based compound combined with an alcohol. The Stach et al. patent, U.S. Pat. No. 4,971,083, entitled Apparatus And Method For Cleaning Solder Paste From Items Associated With Surface Mount Technology Manufacturing, discloses an apparatus for and a method of cleaning solder paste from items associated with surface mount technology manufacturing. Similarly, the Cala et al. patent, U.S. Pat. No. 5,593,504, entitled Method Of Cleaning Solder Pastes From A Substrate With An Aqueous Cleaner, discloses a method of cleaning solder paste from a substrate with an aqueous cleaner. The Barton patent, U.S. Pat. No. 3,723,332, entitled Stabilized Perchlorofluorocarbon Compositions, discloses a combination of a fluorine based compound and an alcohol for use as a cleaning liquid. The following additional patents each disclose features in common with the present invention but are not as pertinent as the patents discussed above. U.S. Pat. No. 5,456,760 to Goehausen, entitled Cleaning Process For Electronic And Electrical Assemblies, U.S. Pat. No. 5,238,504 to Henry, entitled Use Of Terpene Hydrocarbons And Ketone Blends For Electrical contact Cleaning, and U.S. Pat. No. 5,023,010 to Merchant, entitled Binary Azeotropic Compositions Of 1,1,1,2,3,3-Hexafluoro-3-Methoxypropane With Methanol Or Isopropanol Or N-Propanol.
We have found that to avoid the PCBs, the metal mask requires periodic washing steps. Typically, cleaning liquids that clean metal masks are 100% isopropyl alcohol (IPA). However, these cleaning liquids are excessively consumptive due to their high volatility and is not possible to completely remove solder residue from metal masks using 100% IPA cleaning liquids after the solder printing processes. The solder residue in the holes of the metal masks keeps the newly applied solder cream from smoothly passing through the holes. This could lead to poor PCBs having such defects as short circuits, insufficient circuits, lifted circuits, etc., particularly using these 100% IPA cleaning liquids, the above adverse results occur more frequently in the first process PCB just after washing the masks.
When the 100% IPA cleaning liquid is used for washing the metal masks, the solder penetration through the metal masks to the first process PCB is inferior to the subsequently processed PCBs. Only 50% of the holes in the metal masks are in good condition. As the solder printing process progresses, the rate of the poor PCBs decreases but after five or six solder printing processes, the rate of the poor PCBs again increases. In other words, using the previously used cleaning liquids causes a problem such as a high rate of poor PCBs and requires washing steps of the metal masks every five uses of the metal mask. Moreover, there is another problem due to uneven surfaces of the PCBs due to the solder residue after the solder printing process.